In chemical pulp mills the cooking chemicals of a pulping process are recovered from black liquor by firing the black liquor in a recovery boiler alone or together with other “waste” streams. The firing process is exothermic and the released energy is recovered as pressurized superheated steam. The steam energy is recovered in a steam turbine in form of electric power and low-pressure steam for process needs.
Traditionally, energy is produced in a pulp mill by combusting black liquor in a recovery boiler, and wood wastes and bark in an auxiliary boiler. The bark of the wood raw material and the organic substance of generated black liquor together normally supply the entire energy demand of the pulp mill. If more energy is needed in the pulp mill, additional fuel may be purchased. The additional fuel is combusted with the wood bark in the auxiliary boiler. Conventionally, energy is produced in a pulp mill in the following way: a recovery boiler and an auxiliary boiler, wherein waste wood bark from the mill is combusted to generate superheated high-pressure steam. The generated steam is forced through a backpressure steam turbine(s) and the released steam is used to provide heat for the mill. The turbine and a generator connected thereto produce electricity needed to power the mill.
Wood contains small amounts of potassium (K) and chlorine (Cl). These elements remain in the black liquor during cooking. In the recovery boiler, these elements are enriched into the fly ash and increase the corrosiveness of the flue gas especially in the superheater. The corrosiveness of Cl and K increase with temperature. The corrosiveness of Cl and K impose an upper temperature limit on the steam generated in the recovery boiler. This limit for the superheated steam temperature is typically 400° to 490° C., depending on the content of chlorine and potassium. With special materials or with liquors having a very low Cl and K content, steam temperatures up to 520° C. have been used. Because the corrosiveness of Cl and K generally require the temperature of the superheated steam to be held relatively low, the steam pressure is also low. These temperature limitations result in low power yield from the heat generated in the recovery boiler, as compared to normal power boilers fuelled by coal, natural gas or oil.
These temperature limitations on the steam in a recovery boiler are not as strictly valid with bark originating from logs, but the fly ash from bark combustion in a bark boiler may also contain chlorine and potassium. As the sulfur content of bark is very low, potassium reacts in the bark boiler with chlorine and forms KCl K and Cl, which in turn may result in superheater corrosion. Calorimetric flow in bark is also much lower than in the black (“waste”) liquor flow, due to much lower mass flow.
New power cycles developed to replace the traditional recovery boiler and steam turbine cycle have been studied, and pressurized gasification of “waste” liquor and bark appears to have promising possibilities. However, much more development appears to be needed before these technologies have the necessary reliability and performance.
Anyway, efficient systems have not, to the best of our knowledge, been developed for increasing the temperature and pressure of superheated steam produced at a recovery boiler plant of a pulp mill in such a way that no corrosion occurs or the rate corrosion is at acceptable levels. Thus, according to the present invention, a system is provided for increasing the power yield in energy production at a pulp mill so that corrosion problems can be minimized. A further object of the invention is to provide a compact system, in which heat is recovered efficiently when high temperature and high pressure steam is produced.
For achieving the objects mentioned hereinabove the invention relates to a spent liquor recovery boiler system comprising a boiler having a water/steam circulation system having superheaters and a furnace for recovering energy and chemicals from spent liquor combusted therein, the walls of the boiler being formed of a plurality of water-cooled tubes connected to the water/steam circulation system. It is a characteristic feature of the system according to the invention that the system is provided with at least one cavity having walls formed of water-cooled tubes connected to the water/steam circulation system of the recovery boiler, means for combusting a fuel and at least one outlet for discharging combustion gases to the boiler, the interior of the at least one cavity being provided with heat exchanger means for final superheating of the steam generated in the boiler, said heat exchanger means being connected to the superheaters of the boiler.
According to the system of the invention at least one combustion cavity is provided in connection with a recovery boiler for the final superheating of steam produced in the superheater section of the recovery boiler. The system of the invention allows that the steam is heated in the conventional heat transfer sections (i.e. economizers, boiler bank, and superheaters) of the recovery boiler into such a degree that high temperature corrosion does not substantially take place, i.e. below 520° C., optimally 480-500° C., and after that the steam is final superheated to 500-600° C., optimally to 520-560° C. in the combustion cavity, which serves as a final superheater. The recovery boiler system can be provided with one or more final superheater (cavities). The fuel burned in the final superheater/s is such that it does not cause high temperature corrosion.
The walls of the superheater cavity are designed as water-cooled heat transfer surfaces, which are connected to the main water/steam circulation system of the recovery boiler through connection pipes for the incoming water/steam-water mixture and outgoing water/steam-water mixture. Thus, the heat surfaces form a part of the main water system of the recovery boiler. The main water system and consequently the water system of the superheater cavity can be of natural circulation type or forced circulation or so called once-through, the last-mentioned being typical for the highest steam/water pressures. In natural circulation boilers this means that cooling water is fed via downcomers from a drum down to headers feeding the walls of the cavity or cavities, and water-steam mixture from these walls is collected and fed into the drum. The cavity can have separate walls of its own, but part of the walls of the cavity or part of the walls of the cavities can be common with the “conventional” part of the boiler.
The interior of the cavity is provided with heat exchanger means for heat transfer from the combustion gas produced in the cavity to the steam flowing in the heat exchanger means. The heat exchanger means serve as a final superheater for the steam from the superheater section of the recovery boiler.
The optimum location for the cavity is the front wall, which is opposite to the “bullnose” wall, but the cavity can be built on sidewalls too, either as one cavity, or as several ones. The location or locations of the cavity or the cavities can be in the vertical direction anywhere, in relation to the conventional part of the boiler, limited only by the cooling water circulation.
The superheater cavity for the final superheating of the recovery boiler steam is heated by burning fuel. A burner or burners for the fuel are located at the top of the cavity, at the bottom of the cavity, or on the walls of the cavity. The cavity can also be located in a horizontal position, when the most preferred location for the burner or the burners is on one end wall of the cavity.
The combustion air system of the superheater cavity is a part of the combustion air system of the recovery boiler. It may also have a separate air system with an air fan of its own, connection ducts between the fan and the burner(s) and any necessary equipment for the combustion air control.
The superheater cavity for the final superheating of the recovery boiler steam is heated by burning fuel in such a manner that noncorrosive conditions in the superheater cavity are guaranteed. The fuel can be a gas produced by gasifying biomass. The corrosion of heat surfaces can be avoided by additional combustion of sulfurous fuel. Also the cleaning of the gas before the combustion in the superheater cavity guarantees noncorrosive conditions at higher temperatures. Instead of the gas produced from biomass other fuels can be used, e.g. natural gas, LPG, liquefied biomass, methanol, etc. The criterion for the fuel is the noncorrosive nature under the cavity conditions. The fuel combustion in the cavity is normally complete with optimized amount of excess air, but stoichiometric or reducing conditions are also possible, if preferable.
The offgases of the superheater cavity are led into the recovery boiler, preferably to the inlet of the main superheater where they are mixed with the main gas stream coming from the boiler furnace. Other locations for the gas connection are possible as well: the whole area from the lower part of the furnace to the inlet of the economizer. The offgas connection through the boiler wall comprises preferably more than one opening.
The flue gases from the superheater cavity may be used to shape a flue gas flow pattern, from the furnace of the recovery boiler over the bull nose to improve gas distribution and heat transfer into the superheaters, or to generate conditions required for emission control, such as the optimum temperature window for selective non-catalytic reduction (SNCR) to reduce NOx emissions, or for particle size growth to reduce fine particle (size less than 2.5 micrometers) emissions. Another possibility is to use these flue gases for shaping the flow pattern in the furnace to improve mixing, e.g., introducing these gases via vertically located ports, as is described in European patent 668,983 and in WO 02/081971, instead of air or mixed with air. Combustion in the superheater cavity may be performed also under sub-stoichiometric conditions. Reduced gases thus generated can be introduced into flue gases from the furnace to reduce nitrogen oxide emissions in the flue gases coming from the furnace.
By means of the invention the pressure and the operating temperature of the steam recovered from the waste liquor recovery process is increased by means of the superheater cavity, whereby the overall electrical efficiency of the plant is improved, i.e. more power is generated by the heat recovered in the steam. The system according to the present invention is preferably used in connection with the process described in international patent application PCT/FI03/00358.
During the past years waste liquors from pulping were burned in a separate, refractory lined combustion chamber, which was connected to a recovery boiler. Flue gases flowed from the combustion chamber into the furnace of the recovery boiler, which was equipped with oil or gas burners or with grates for bark and coal etc. In these boilers the separate combustion chamber was used to stabilize combustion, not to superheat steam to the final high temperature from the superheaters of the upper part of the furnace of the boiler. An essential feature of the present invention is that the combustion cavity and the recovery boiler have a common water circulation system. U.S. Pat. No. 2,606,103 describes a system, in which the walls of a separate furnace are cooled by air.